Japanese Patent Application Laid-Open (kokai) No. 11-185928, hereinafter “Patent Document 1,” discloses a conventional spark plug. The spark plug includes a ground electrode. The ground electrode has a proximal end section fixed to a metallic shell, a bent section formed integral with the proximal end section, and a distal end section formed integral with the bent section and adapted to form a spark discharge gap in cooperation with a center electrode.
The ground electrode is configured to have a core portion extending from the proximal end section toward the distal end section via the bent section, a heat transfer portion located externally of the core portion and extending from the proximal end section toward the distal end section via the bent section, and an external layer located externally of the core portion and the heat transfer portion and extending from the proximal end section to the distal end section via the bent section. As viewed on a section taken orthogonally to a direction along which the ground electrode extends, the centers of the external layer, the heat transfer portion, and the core portion coincide.
The external layer is formed from a nickel-based alloy, which serves as a first metal; the heat transfer portion is formed from copper, which serves as a second metal; and the core portion is formed from pure nickel, which serves as a third metal. The nickel-based alloy used to form the external layer is excellent in heat resistance and corrosion resistance. Copper used to form the heat transfer portion has a thermal conductivity of 390 W/m·K, which is higher than that of the nickel-based alloy. Pure nickel used to form the core portion has a Vickers hardness Hv of 125, which is higher than the Vickers hardness Hv of copper (Hv 75). Copper used to form the heat transfer portion has a linear thermal expansion coefficient of 1.65×10−5/° C., which is higher than the linear thermal expansion coefficient of the nickel-based alloy (1.34×10−5/° C.) and the linear thermal expansion coefficient of pure nickel (1.30×10−5/° C.).
The thus-configured conventional spark plug is mounted on an engine and repeatedly discharges between the center electrode and the ground electrode under a high temperature condition.
During the repeated discharge of the spark plug, by virtue of excellent thermal conductivity of the second metal used to form the heat transfer portion, the heat transfer portion effectively conducts heat from the distal end section to the proximal end section. That is, in the spark plug, by virtue of excellent heat transfer performance of the heat transfer portion, an increase in temperature of the distal end section is restrained, so that excellent durability can be exhibited.
Meanwhile, because of the high linear thermal coefficient of the second metal used to form the heat transfer portion, the ground electrode of the spark plug tends to rise under a high temperature condition. Upon occurrence of the rising of the ground electrode, the discharge gap between the ground electrode and the center electrode changes, causing an adverse effect on characteristics. Thus, in the spark plug, such rising of the ground electrode is restrained through adjustment of the thicknesses of the heat transfer portion and the external layer. Conceivably, the reinforcement effect of the core portion implemented by the third metal used to form the core portion being higher in hardness than the second metal used to form the heat transfer portion contributes to restraint of the rising of the ground electrode.